Pattern measuring method

ABSTRACT

A pattern measuring method calculates an average pattern shape from a plurality of the same patterns appearing within an image captured using an electron microscope, and compares pattern information at each measuring position with average pattern information to determine roughness.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/066,219,filed Feb. 28, 2005, now U.S. Pat. No. 7,180,062 which is based onJapanese Patent Application No. JP 2004-092987 filed Mar. 26, 2004, thecontents of which applications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a pattern measuring method for use witha scanning electron microscope which scans a specimen with electronbeams and captures information on the surface of the specimen in theform of image from the amount of a secondary electron signal emittedfrom the specimen as a result of the scanning, and more particularly, toa method of stably measuring an average line width of a pattern.

With the increasing miniaturization of semiconductor devices, the edgeroughness of a pattern has become a problem. The edge roughness has beeninvestigated in regard to how it occurs, and attempts have been made fora management based on the roughness value, and a management based on anaverage measured value including the edge roughness used as anindicator.

FIG. 2 shows an example of a conventional line width measuring method.An edge detecting range is set for a pattern under measurement. Theprofile of an image is created from image information, and the positionsof edges are determined using information on the waveform of adifferentiated version of the profile. The line width is determined frominformation on the left and right edges (in the same orientation ofadjacent patterns when in a pitch measurement). A portion correspondingto a convex edge of the pattern appears in white in a SEM image. Even astraight line by design results in an undulate form, as indicated by ablack wave line in the figure. The edge roughness is defined to bevariations in the noted right or left edge, while a width roughness isdefined to be variations in the line width measured a plurality of timesin the vertical direction.

The prior art related to the foregoing pattern measurement is disclosed,for example, in JP-A-2003-37139.

The width roughness will be described with reference to FIG. 3. Themeasurement of a line width involves setting edge detecting areas alongthe two edges, as indicated by black bold lines in FIG. 3, and detectingedges within the edge detecting areas. Then, the distance between theedges is defined to be a line width in that area. However, whenroughness exists as shown, the line width largely differs at a measuringposition A and at a measuring position B in FIG. 3.

Possible causes for the roughness can include variations attributable tothe measuring method other than variations in the shape itself. FIGS. 4Aand 4B show exemplary profiles at the measuring position A and measuringposition B in FIG. 3, respectively. Generally, for making measurementsat a plurality of measurement points, a common parameter is set andapplied to each pattern under measurement. Assume that a line width ismeasured with thresholds which are 30, 50, 70% of the difference betweenthe highest point and the lowest point in a profile. When the patterndoes not include any local step as shown in FIG. 4B, measured valuesincrease at a constant rate in the order of 30%, 50% and 70%. In otherwords, there is a linear relationship between the threshold and themeasured value. On the other hand, when a profile include local steps asshown in FIG. 4A, measured values irregularly vary depending on which of30%, 50% and 70% threshold should be used. In other words, measuredvalues largely vary depending on a set threshold. Since such steps canlocally and sporadically appear in a pattern, measured values largelyvary depending on an area used for the measurement and on a setthreshold, even in a line pattern as shown in the example.

Consider that the foregoing measurement is applied to a line patternwhich has local edge roughness as shown in FIG. 3. Portions representedby white bold lines are called “white bands” which correspond to theedges of the line, and include global pattern roughness. On the otherhand, portions indicated by black thin lines represent the result ofmeasurement. The latter is indicated by black thin line in order todistinguish the global pattern shape from the local roughness.

The pattern having the roughness as shown is measured at the measuringpositions A and B. The luminance value is accumulated in a rectangulararea (measuring area) around the measuring position A in the verticaldirection to create its profile, and information on the edges at themeasurement point A is acquired from the profile resulting from theaccumulation. Information on the edges at the measuring position B isacquired in a similar manner. Consider now that the line width ismeasured from the distance between the respective edges at the measuringpositions A, B. From a viewpoint of a stable measurement of the linewidth in the line pattern, it is desired to be able to measure the linewidth W, represented by the spacing between the white lines in FIG. 3,from which the influence of the roughness is omitted. However, theaforementioned method is adversely affected by the local roughness, sothat a line width W_(A) is measured at the measuring position A, while aline width W_(B) is measured at the measuring position B, thuspresenting largely differing results of measurement depending on theposition set for the measurement. While the roughness is seeminglycaused by a process, a material and the like, correct causes cannot havebeen so far identified, so that the global line width can be preferablymeasured separately from the roughness.

Also, conventionally, when an average line width is measured for a lineor a space pattern image including edge roughness, the line width islocally measured at multiple positions, and measured values areaveraged, thus implying problems of an extended processing time,instable calculated values, and the like.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a measuring methodwhich is capable of stably measuring a line width in a simple mannerseparately from local roughness.

Conventional measuring techniques for measuring a line width in apattern image involve setting a certain target, locally detecting edgeswithin the range of the target, and measuring the line width based oninformation resulting from the local edge detection, so that theinfluence of the local edge roughness causes variations in the measuredvalues. The present invention, on the other hand, makes a measurement onan average image of a plurality of the same patterns within the samescreen, and can therefore measure the local edge roughness and globalroughness, which can cause the variations, and variations in roughnessin the screen. Also, when the measurement is specified for the globalroughness, the local roughness of an image is averaged by superimposinga plurality of patterns within the same screen, thus reducing errors dueto a particular measuring method.

To solve the foregoing problem, the pattern measuring method of thepresent invention measures a line width by capturing an image at ascaling factor which permits a plurality of patterns under measurementto appear within the same screen, and measuring a line width androughness for an arbitrary pattern within the captured image usingperiodic information and shape information such as a line, a space, ahole, and the like under measurement. The pattern measuring method alsocalculates an average value for the pattern measurement from the resultsof measuring patterns captured at a plurality of different locations,and calculates roughness information possessed by each of the pluralityof patterns within the screen.

Specifically, the pattern measuring method of the present inventionincludes the steps of capturing an image of a specimen at a scalingfactor which permits a plurality of patterns under measurement to appearwithin a field of view, creating a plurality of profiles in a shapemeasuring area corresponding to each of the plurality of patterns undermeasurement, calculating an average profile by averaging the pluralityof profiles created in all the shape measuring areas corresponding tothe plurality of patterns under measurement, and determining a shaperoughness for the shape measuring area corresponding to each patternunder measurement using the average profile and the plurality ofprofiles created in the shape measuring area of each pattern undermeasurement. When the pattern under measurement is a line or a spacepattern, the width of a line or a space from the profile can becalculated from the profiles, and an average width of the line or thespace is calculated from the average profile to determine widthroughness as the shape roughness.

The average profile can be calculated by averaging the plurality ofprofiles created in the shape measuring area corresponding to each ofthe plurality of patterns under measurement. Alternatively, the averageprofile can be calculated by accumulating luminance values in the shapemeasuring area for each of the pattern under measurement in a directionalong the line or the space to create a accumulated profile, andaveraging the accumulated profiles.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an exemplary configuration ofa scanning electron microscope for conducting a miniature pattern testaccording to the present invention;

FIG. 2 is a diagram showing an exemplary conventional line widthmeasuring method;

FIG. 3 is a diagram for explaining roughness;

FIGS. 4A and 4B are diagrams showing exemplary profiles;

FIGS. 5A to 5D are diagrams showing patterns which are assumed formeasurements;

FIG. 6 is a flow chart illustrating the flow of a pattern measurement;

FIG. 7 is a flow chart illustrating a measuring procedure; and

FIGS. 8A and 8B are diagrams illustrating an example of a measured lineand space pattern.

DETAILED DESCRIPTION OF THE INVENTION

In the following, one embodiment of the present invention will bedescribed with reference to the accompanying drawings.

FIG. 1 is a schematic diagram illustrating an exemplary configuration ofa scanning electron microscope for conducting a miniature pattern testaccording to the present invention. In an electron gun 3, a heatingfilament 2 is heated with a high voltage (500 volts or higher) togenerate an electron beam 8. Subsequently, the electron beam 8 drawn outby a Wehnelt 4 is accelerated by an anode 5. This electron beam 8 isconverged by a condenser lens 6, and is scanned in an arbitrarydirection by a deflection signal generator 16 which is composed of adeflection coil 7, a scaling factor varying resistor 12, and a scanningpower supply 16. Further, the electron beam 8 is focused by an objectlens 9, and one-dimensionally or two-dimensionally scanned on a specimen11 placed in a specimen chamber 10. Miniature patterns are engraved onthe specimen 11. The irradiation of the electron beam 8 causes secondaryelectrons to be generated near the surface of the specimen 11 in anamount in accordance with the shape of the specimen 11. The secondaryelectrons are detected by a secondary electron detector 17. The detectedsecondary electrons are amplified by an amplifier 18 for transformationinto a luminance modulation signal of a CRT 14. The CRT 14 is insynchronism with the deflection signal generator 16, so that theluminance modulation signal reproduces a secondary electron imagegenerated by the electron beam 8 irradiated in synchronism from thesurface of the specimen 11. Information on the surface of the specimencan be acquired through the foregoing procedure.

FIGS. 5A to 5D illustrate exemplary patterns which are assumed formeasurement, and FIG. 6 illustrates the flow of a pattern measurement.As illustrated in FIGS. 5A to 5D, assume that a plurality of patternsunder measurement repeatedly appear in the screen. As long as aplurality of patterns under measurement repeatedly appear in the screen,there is no particular limitations in the shape of pattern.

FIG. 6 illustrates a procedure for registering a measuring positionwithin the screen. A scaling factor is set such that a plurality ofrepeated patterns under measurement appear within the same screen. Next,a registered position is set within the screen (401), and the positionis registered (402). A confirmation is also made at this step as towhether or not a registered pattern has been correctly registered. Whena portion without patterns is intentionally registered, thisregistration is treated as failed. Next, a detection is made as towhether or not a plurality of patterns are included in the screen (403).The registration procedure is terminated when a plurality of patternsare detected, whereas the scaling factor is reduced when a plurality ofpatterns are not detected (405), followed by a repetition of theexecution from step 401 to step 404. Here, if a plurality of patterns donot appear within the screen even at a reduced scaling factor, theprocedure is terminated, regarding as an error. When foreign substancesor the like are registered in a region in which no patterns exist, theprocedure is terminated, regarding as an error, because the samepatterns are not found even at a reduced scaling factor.

Next, FIG. 7 illustrates a prescription-based measuring procedure formaking automatic measurements at a plurality of measuring positionswithin a wafer. The prescription is registered with coordinateinformation indicative of general measuring positions, a pattern(template) for finding a field of view through pattern matching afterthe field of view has been moved to the coordinates, relative positionalinformation of a measuring area to the template, a measurement mode (aparameter for the measurement), and the like. There are two types ofregistered templates which include a template for finding the field ofview at a low scaling factor, and a template for finding the field ofview at a high scaling factor.

First, a prescription file which registers a pattern is read (701).Then, a registered pattern is read out (702), and is chosen for use inthe pattern detection. Next, the field of view is moved to a measuringposition (703). The same pattern as the template read at step 702 isdetected at the measuring position (step 704). After the patterndetection, a profile is created for measuring positions based on theimage (705). When the processing at step 704 and 705 has not beencompleted for all of a plurality of patterns found within the screen,the field of view is moved to the position of the next pattern (707),followed by a repetition of the processing from step 704 to step 706.After the profiles have been created for the plurality of patternswithin the screen, an average profile is created (708). The averageprofile may be created by averaging all the created profiles, or byaccumulating luminance values in the measuring area in the verticaldirection (direction along the line) for each pattern under measurementto create a accumulated profile, and averaging the accumulated profilewith the plurality of patterns.

In a line and space pattern illustrated in FIGS. 8A, 8B, a profile iscreated in each of three measuring areas A, B, C, for example, asillustrated in FIG. 8A. The measuring areas A, B, C are defined longerin the vertical direction as indicated by black frames in FIG. 8A, and aplurality of profiles are created at regular intervals within the blackframes. When edges are detected using the thus created profiles, avertical pattern shape can be measured. Two black thin lines shown inFIG. 8B represent edge positions, i.e., a pattern shape, including localroughness measured, for example, in the measuring area A. Also, anaverage profile is calculated in the measuring areas A, B, C bysuperimposing all profiles created in the measuring areas A, B, C. Then,an average shape for a pattern averaged in the measuring areas A, B, Cis found from the average profile. White lines in FIG. 8B represent theaverage shape of the pattern thus found. The average shape and roughnesscan be measured with a higher accuracy by statistically comparing thepattern shape measured in the measuring area A or the measuring area B,C, represented by the two black thin lines, i.e., the pattern shape(line width) found from the plurality of profiles created in themeasuring area with the average pattern of all the patterns representedby white lines.

In regard to the measuring areas A, B, C, when they are identical inshape but different in design width, or when they have been previouslyknown to be in a bad forming condition, similar calculations may be madeby setting a contribution ratio to the average profile.

As described above, it is possible to measure variations in measurementdepending on the pattern position by comparing a profile at each ofmeasuring positions with an average profile. According to the method ofthe present invention, since an image can be captured at a lower scalingfactor, damages to patterns can be reduced. Also, since a plurality ofpatterns can be measured from a single image, the method of the presentinvention helps improve the throughput. Further, since an averagepattern is detected from a plurality of patterns identical in shape froma single image, it is possible to alleviate the influence of local edgeroughness which can cause variations in measured values, and to quantifythe tendency of the appearance of edge roughness by comparing theaverage pattern with each of pattern shapes at the respective positions.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

1. A method of measuring a shape of repetition patterns using a scanningelectron microscope, comprising the steps of: setting a scaling factorof said scanning electron microscope which permits a plurality ofrepetition patterns to appear within a field of view; measuring shape ofthe repetition patterns included in the field of view and calculating anaverage value of the measured shape; and comparing the average valuewith shape roughness for the repetition patterns.
 2. A pattern measuringmethod according to claim 1, wherein: said patterns under measurementinclude a line or a space pattern, and said step of measuring the shapeincludes: measuring width of a line or a space from the shape; anddetermining an average width of the line or the space from the averagevalue to determine width roughness as the shape roughness.
 3. A patternmeasuring method according to claim 2, wherein said step of measuringthe shape includes: accumulating luminance values in a shape measuringarea for each of the patterns under measurement in a direction along theline or the space to create an accumulated profile, and averaging theaccumulated profiles.
 4. A pattern measuring method according to claim1, wherein said average value to be calculated in said step of measuringthe shape is determined by calculating an average value of valuesmeasured at a plurality of positions in a shape measuring areacorresponding to each of the plurality of patterns under measurement. 5.A scanning electron microscope comprising: an electron source; adeflector for two-dimensionally scanning an electron beam emitted fromsaid electron source; and a processing unit for measuring dimension ofpatterns on a sample; wherein said processing unit measures a shape of aplurality of repetition patterns included in a scanning area of theelectron beam, calculates an average value of the measured shapes, andcompares the average value with shape roughness for the repetitionpatterns.
 6. A scanning electron microscope according to claim 5,wherein: said patterns under measurement include a line or a spacepattern, and said processing unit measures width of the line or thespace from the shape, and determines an average width of the line or thespace from the average value to determine width roughness as the shaperoughness.
 7. A scanning electron microscope according to claim 6,wherein said processing unit accumulates luminance values in thescanning area of the electron beam for each of the patterns undermeasurement in a direction along the line or the space to create anaccumulated profile and to average the accumulated profiles.
 8. Ascanning electron microscope according to claim 5, wherein saidprocessing unit determines the average value by calculating an averagevalue of values measured at a plurality of positions in the scanningarea of the electron beam corresponding to each of the plurality ofpatterns under measurement.
 9. A scanning electron microscopecomprising: an electron source; a deflector for two-dimensionallyscanning an electron beam emitted from said electron source; and aprocessing unit for measuring dimension of patterns on a sample; whereinsaid processing unit determines measurement values of a plurality ofrepetition patterns included in a scanning area of the electron beam andcalculates an average value of the measurement values on the basis ofdesign width of the plurality of patterns or forming condition of theplurality of patterns.
 10. A scanning electron microscope according toclaim 9, wherein said processing unit compares the average value withroughness for the repetition patterns.